1. Field of the Invention
The present invention relates to an improvement of an aperture controlling system for obtaining an optimal aperture, at the time of flash photographing, from a distance to an object, a guide number (at full light emission) of a strobodimmer, a film photosensitivity and other conditions.
2. Description of the Related Art
If F be an aperture and D a distance to an object, then a guide number GNo of a stroboscope will be: EQU GNo=F.times.D (1)
Further, if a quantity of light is denoted by P, a brightness Eo on an object surface will be expressed by Eq. (2) from it being in inverse proportion to the square of the distance D: ##EQU1##
Against the brightness Eo of the object surface, a brightness E of a film surface is in inverse proportion to the square of the aperture F, therefore ##EQU2## The brightness E of the film surface must be weakened according as a film photosensitivity gets high, therefore if the film photosensitivity is denoted by S, the brightness E of the film surface must be inversely proportional to the film photosensitivity S. Thus, ##EQU3## If Eq. (4) is subjected to a logarithm to the base 2, then all can be represented by APEX values. EQU log.sub.2 S+log.sub.2 P.varies.log.sub.2 F.sup.2 +log.sub.2 D.sup.2 =log.sub.2 (GNo).sup.2 ( 5)
In Eq. (5), if log.sub.2 (K.sub.1 S)=Sv, log.sub.2 (K.sub.2 P)=Pv, log.sub.2 F.sup.2 =Av, log.sub.2 D.sup.2 =Dv, log.sub.2 (GNo).sup.2 =Gv (K.sub.1, K.sub.2 being constants), then Eq. (5) will be simplified as: EQU Sv+Pv=Av+Dv=Gv (6)
Sv and Av being defined formulaically, Pv, Dv and Gv will be defined here.
Pv is the quantity of light of a stroboscope, and its magnitude is decided generally with a guide number when ISO is 100, therefore it is defined by GNo when ISO is 100. Dv and Gv are then defined in a .sqroot.2 sequence as in the case of Av. When defined as FIG. 13 on the premise above, Eq. (6) given above may hold accordingly.
When using a manual stroboscope (not for dimming), the guide number will be obtained in consideration of the film photosensitivity. Then, the distance is obtained from a lens distance ring as focusing the camera, and the aperture is calculated according to Eq. (1) to manual setting. Like result will be obtained from replacing with APEX values to a calculation on Eq. (6). Since an operation of the manual stroboscope involves a complication, there prevailing of late is an autostroboscope (dimmer type) which is ready for operation simply from setting the aperture. The construction is then such that the aperture Av is set by a photographer and the guide number Gv changes according to the distance Dv in Eq. (6). That is, it is ready for photographing by means of GNo coming less than the maximum GNo (at full light emission). In the case of strobodimmer, Eq. (6) can be rewritten as: EQU Av+Dv.ltoreq.Sv+Pv (max)=Gv (max) (7)
and further as: EQU Av.ltoreq.Sv+Pv (max)-Dv (8)
This is a basic APEX formula of the strobodimmer.
However, an unavoidable defect is such that Dv coming over a certain value or a certain distance may lead to a short of exposure.
Now, therefore, the photographer is inclined to expand a photographable range by using, as far as possible, an aperture on the open side. However, in this case where the object is not brought into accurate focusing before photographing, a photograph will be obtained out of focus, or since a dimming performance of the stroboscope is limited in a short distance photographing, an overexposure may result on the photograph.
To solve the aforementioned disadvantages, there is disclosed a strobodimmer system wherein a stroboscope preemits the light on the first stroke of a release button and detects the light reflected from a main object to measure the distance, and a camera operates a minimum aperture (on the most stopped-down side) ready for strobodimming according to a distance information from the stroboscope, a guide number (at full light emission) of the stroboscope and a film photosensitivity, and the camera is adjusted to the aperture thus operated in, for example, U.S. Pat Nos. 4,484,807, 4,501,481, 4,547,057, 4,610,525, 4,572,644, 4,569,580. Thus an out-of-focus condition will not be caused thereby, a proper exposure will be secured even at a short distance photographing, and a fairly long distance photographing can be realized.
Meanwhile, it is advantageous for continuous emission of the light that an excessively large dimming GNo be not used in a strobodimmer in consideration of recycle time. The aforementioned strobodimmer system is therefore not satisfactory in this respect.
In such strobodimmer system, furthermore, when a stroboaperture priority mode or a stroboshutter priority mode is set, if an aperture computed from an extraneous light luminance correspondingly to the aperture set by a photographer or the shutter speed comes toward an open side from the minimum aperture computed according to a distance information from the stroboscope, then the aperture set by the photographer may be a good one to set on the camera, however, if it comes toward a stopped-down side, the minimum aperture computed according to the distance information from the stroboscope will be the camera aperture, and thus there is a trend prevailing that the stroboscope has priority rather than an intention of the photographer.